1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device having a backlight which includes a light guide plate and light emitting diodes.
2. Background Art
Conventionally, the liquid crystal display device has been used as a display part of portable electronic equipment, for example, because of a reduced thickness, a reduced weight and small electric power consumption.
The liquid crystal display device is not of a self-luminous type and hence, a planar lighting device which is referred to as a backlight is arranged on a back surface of a liquid crystal display panel, for example. Although a conventional backlight uses a fluorescent lamp such as a cold cathode ray tube as a light source (light emitting element), light emitting diodes (LED) have been popularly used as a backlight recently.
The backlight is roughly classified into a direct backlight and an edge-light-type (light-guide-plate) backlight. In the direct backlight, a light source is arranged on a back side of a display region of a liquid crystal display panel. On the other hand, in the edge-light-type backlight, a light guide plate is arranged on a back side of a display region of a liquid crystal display panel. The light guide plate radiates light from a light source in a planar shape, and radiates the light to the liquid crystal display panel.
The light guide plate is formed using a light transmitting resin, for example. Light which is incident on the light guide plate from the light source propagates in the light guide plate. Reflecting/scattering members such as grooves, projections or prints are formed on the light guide plate. Light which propagates in the light guide plate is reflected or scattered by the reflecting/scattering members, and is radiated toward a liquid crystal display device side.
With respect to the light guide plate used in the backlight of the liquid crystal display device, it is desirable that a light quantity (surface brightness) of light radiated from the light guide plate is uniform and, at the same time, the polarization direction is uniform over the whole surface of a light radiation surface of the light guide plate.
With respect to the conventional backlight, there has been known a method in which, to make the surface brightness of light radiated from the light guide plate uniform, apex angles of prisms formed on a light radiation surface of the light guide plate are changed corresponding to a distance from a light source. This method is proposed in JP-A-2004-192909, for example.